Carcinogenesis is a multistage process that progresses via a series of mutations in specific oncogenes and tumor suppressor genes. Evidence suggests that spontaneous mutation rates alone cannot account for the level of mutations observed in most tumors. A hypothesis for the elevated mutation rates in tumors is that tumor progression may be aided by the acquisition of a "mutator" phenotype that produces genomic instability. Gene amplification, microsatellite alterations, and chromosome aberrations have been observed in many tumor types. However, there has been little characterization of instability in the form of gene mutations, especially in vivo. Transgenic mice with a lacI target gene allow rapid quantitation and sequencing of mutations in nearly any tissue or tumor. This system is useful for studying genomic instability because, unlike oncogenes and tumor suppressor genes, mutations in the nontranscribed lacI gene confer no selective growth advantage or disadvantage. Preliminary studies show that mouse liver tumors have an elevated lacI mutation frequency consistent with the hypothesis of cancer-related genomic instability. The objective of this proposed research is to use transgenic lacI and p53 +/- mice to characterize genomic instability in tumors in response to genotoxic and nongenotoxic environmental carcinogens. The specific aims will be to 1) determine the level of genomic instability in lacI transgenic mouse liver tumors that are spontaneous or induced by treatment with genotoxic and nongenotoxic agents, alone or in combination; 2) characterize the nature of genomic instability in tumors by characterizing mutational spectra at the lacI gene; 3) correlate genomic instability at the lacI gene with tumor stage, microsatellite instability, activation of Ha-ras, alterations in p53, and cell proliferation; 4) determine the role of p53 in cancer-related genomic instability using lacI/p53+/- transgenic mice; and 5) determine whether genomic instability and other alterations observed in liver tumors are present in tumors at other sites. A large database of DNA sequence and molecular changes associated with carcinogenesis will be develops The information obtained should improve the understanding of the role of genomic instability in cancer. Such information may lead to novel therapeutic targets and diagnostic tools.